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A serious, scholarly and (we hope) inoffensive guide to set the record straight on the sizing conventions of bras. They are used incorrectly throughout the wiki itself as well as in various media.
Most important point: The alphabetical part of the size is not an objective measure of breast volume or mass, nor is it a straight ratio or proportional measurement. What actually is will become slightly clearer below.
In the United States, a bra size is determined thus: Measure the circumference of the chest below the breasts in inches, round to the nearest inch, then add 4 if it's even or 5 if it's odd to account for the overlap at the ends; that's X, the strap size. Measure the circumference of the chest over the breasts at the widest point, and round off; that's Y. The letter, aka the cup size, is Y minus X, converted to a letter. The center of a properly fitted bra will touch the wearer's chest. So, a woman with a 34-inch chest would need a 38 strap, and if her over-breast measurement is 41 inches, she'd need a C cup: 38C. A woman with a 41-inch chest and a 47-inch over-breast measurement would need a 46B. A robust woman with a 54-inch chest and 63-inch outside measurement would wear a 58DD.
Manufactured bras are made only to even-numbered strap sizes, but the adjustment hooks can bring it in one inch pretty easily. Custom jobs can, of course, be almost any size. Sizes above D either are rated as E, F, G and so on, or use multiple D's, so D, DD, DDD and so on. Most commonly, the scale goes D, DD, F, G, etc., up as far as J or K, but it varies by brand.
A bra that is one letter smaller and one number bigger (or vice versa) is a "sister-size". So, a 38D and a 40B would both fit around the same woman, but the center of the 40B wouldn't make contact, and wouldn't properly support her.
Other countries use the same formula, but do it in centimeters. Since the centimeter is about two-fifths of an inch, cup sizes in Japan (for example) tend to be later in the alphabet than equivalents in the United States, for a woman of the same size. This, combined with putting big implants on a smaller ribcage, is why Japanese AV actresses often have I or J cup sizes listed on the front cover of their DVD. (Or So I Heard.)
Upshot being, the actual geometry of bra sizing tends to scale poorly, especially when one is dealing with women or female humanoids outside the average size range. The Square-Cube Law doesn't really work in our favor here, since the operative dimension is the circumference of a curve fitted to a non-convex two-dimensional cross section of an allegedly three-dimensional object — though we're loath to call women either two-dimensional or objects. She-Hulk might have massive cubic displacement, objectively speaking, but her ribcage is also quite massive (Depending On The Artist), so her letter size could be deceptively low. At the same time, a more slender or youthful character like Jubilee with a small ribcage and seemingly small breasts ( for a comic book, anyway) might be early in the alphabet.
Don't feel too bad if you screw it up. Most of the real women in the real world are wearing the wrong size bra in Real Life — really! The average American woman is actually a D-cup, but since many women think that the higher letters are reserved for women with implants, they buy a sister-size that sounds more reasonable, and don't get the fit they really need.